Fuel cells that utilize renewable fuels and oxygen (or hydrogen peroxide) as oxidant are environmentally friendly devices that generate electricity and water as the products of the oxygen reduction reaction (ORR), and as such these fuel cells don't produce the toxic reaction products produced by other more traditional energy sources. Polymer electrolyte membrane fuel cells (PEMFCs), a type of fuel cell being developed for several applications including automotive, require electrocatalysts to oxidize hydrogen at the anode and to reduce oxygen to water at the cathode through the ORR thereby providing desirable current. Expensive conventional catalysts, such as platinum and platinum based materials, result in high manufacturing and production costs. For instance, the manufacturing costs of the fuel cell electrodes for an electrically powered car were estimated in 2006 to be $50-$100 per kW. For an electrically powered car with a power output of, e.g., 80 kW, manufacturing costs of $4000-$8000 for the electrodes alone can be expected.
In an effort to maintain or improve current density outputs of fuel cells while decreasing production costs, attempts have been made to replace platinum based electrocatalysts of the ORR with cost-effective non-platinum group metal (non-PGM) catalysts. Non-PGM catalysts incorporating transition metals such as iron and cobalt are among the possible candidates. These catalysts are active toward the ORR and exhibit selectivity toward the four electron oxygen reduction pathway.
Early non-PGM catalyst research has focused on materials produced by pyrolyzing metal-N4 macrocyles adsorbed on carbon black in an inert atmosphere. Electrocatalysts for the ORR have also been developed by pyrolyzing a metal precursor (e.g., cobalt acetate), carbon black and a nitrogen precursor such as polyacrylonitrile in inert atmosphere. More recently, chelating agents have been encapsulated within the cage structure of a metal organic framework (MOF) which is subsequently thermally treated to remove the metal node. The heat treatment transforms the structure into a conductive and highly active carbon catalyst structure while maintaining or enhancing porosity. Unfortunately, methods that have been developed thus far to prepare non-PGM catalysts require multiple synthesis and processing steps such as polymerization reactions with or without sacrificial templating materials, acid leaching, or formation of complex precursor materials. These methods also include one or more high temperature processing steps sometimes including acid leaching between thermolysis steps.
What are needed in the art are low cost electrocatalyst materials for the ORR that do not require the inclusion of any precious metals, i.e., non-PGM ORR electrocatalysts. Moreover, what are needed are economical formation methods for the electrocatalysts. For instance, a single-step formation process for a conductive microporous support that includes accessible active sites in the support structure would be of great benefit. Additionally, a low temperature formation process for non-platinum group metal oxygen reduction reaction electrocatalysts would be of great benefit.